When drilling boreholes in the oil and gas industry, the nature of the underground formations surrounding the borehole is typically obtained by making physical and/or chemical measurements with tools (often called sondes) located in the borehole, the measurement responses of which are affected by the properties of the formations. A series of such measurements made along the length of the borehole is known as a log and one common form of log is that of a measurement relating to the electrical resistivity of the formation. Resistivity logging techniques are typically classified as either laterologs or induction logs.
Induction logs use an electric coil in the sonde to generate an alternating current loop in the formation by induction. The alternating current loop, in turn, induces a current in a receiving coil located elsewhere on the sonde. The amount of current in the receiving coil is proportional to the intensity of current loop, hence to the conductivity (reciprocal of resistivity) of the formation. Multiple transmitting and receiving coils can be used to focus formation current loops both radially (depth of investigation) and axially (vertical resolution). Known types of induction logging sondes are the 6FF40 sonde which is made up of six coils with a nominal spacing of 40 inches, and so-called array induction tools. These comprise a single transmitting coil and a large number of receiving coils. Radial and axial focusing is performed by software rather than by the physical layout of coils.
Induction tools date back to the late 1940s and have so far been based on an assumption of negligible dielectric effects in induction-tool design, processing and interpretation. Induction tools have become the industry mainstay resistivity-saturation measurement since their introduction in the 1950s. The fundamental feature is a direct measurement of the electric conductivity deep in the formation. The basic measurement is mostly unperturbed by any parasitic effects and therefore is quite easily interpreted.
Recent developments of induction tools have provided accurate measurements of in-phase and quadrature signals. The quadrature signal is used to provide a skin-effect correction to the in-phase signal. Traditionally, induction-tool processing and interpretation neglects dielectric effects completely. The present invention re-introduces dielectric effects into induction-tool processing and proposes two simple inversion algorithms that can be used to determine a dielectric permittivity and electric conductivity from the in-phase and quadrature signals simultaneously.